Methods and arrangements for carbon dioxide storage in subterranean geological formations

ABSTRACT

The invention concerns an arrangement for injecting CO 2  in a supercritical state into a subterranean geological formation, said arrangement comprising: a conduit having a proximal portion and a distal portion, at least part of said distal portion extending in a substantially horizontal direction; multiple openings being provided in said distal portion of said conduit for injection of CO 2  into said geological formation; wherein said multiple openings are provided with outflow limiting means for limiting the flow rate of CO 2  through respective said opening into said geological formation. The invention also concerns a method for storage of CO 2  by said arrangement.

FIELD OF THE INVENTION

The invention relates to methods and arrangements for carbon dioxide (CO₂) storage in subterranean geological formations. In particular, the invention relates to arrangements and methods which maximize the amount of CO₂ storable in a particular formation, thus increasing the usable capacity of a respective reservoir.

BACKGROUND

Several studies indicate that CO₂ and other “greenhouse gases” are responsible for the global climate change, which i.a. includes an increase of the average ambient temperature. This phenomenon is generally referred to as “global warming”. To prevent or reduce global warming, extensive research is conducted for identifying strategies of reducing net carbon dioxide emissions. This includes the search for more energy efficient power plants, vehicles and airplanes, but also includes the concept of carbon dioxide sequestration in subterranean geological formations, such as in depleted oil, gas reservoirs, and abandoned or non minable coal deposits. Permanent CO₂ storage is also envisioned in aquifers, such as, e.g., water-saturated underground porous rock formations. It is generally believed that the permanent storage of CO₂ in subterranean geological formations can make an important contribution to the reduction the CO₂ concentration in the atmosphere.

An extensive review of the existing technology is provided in the IPCC Special Report on Carbon Dioxide Capture and Storage (IPCC, 2005, Bert Metz et al. (Eds.), Cambridge University Press, UK; also available from http://www.ipcc.ch/publications_and_data/publications_and_data_reports_carbon_dioxi de.htm).

CO₂ storage in subterranean geological formations has been practiced in several industrial scale projects, all reviewed in the above IPCC publication. These projects employ, to a large extent, conventional drilling and completion technology to inject large quantities of CO₂ (1 to 10 MtCO₂ per year) into subterranean reservoirs.

CO₂ injection into a subterranean geological formation for Enhanced Oil Recovery (EOR) has been applied in the Rangely EOR project, Colorado, USA. A sandstone oil reservoir has been flooded with CO₂ by a water-alternating-gas (WAG) process since 1986. In this project, CO₂ in a supercritical state is used to extract additional amounts of oil from the otherwise exhausted oil fields in a tertiary oil recovery process. By the end of 2003, 248 active injectors of which 160 are used for CO₂ injection and 348 active producers were in use in the Rangely field. Injection of CO₂ occurs through slots in multiple vertical wells. Vertical wells have a relatively low injection capacity; therefore a great number of such wells are needed. This technology is thus laborious and expensive.

The Sleipner Project, operated by Statoil in the North Sea, is a commercial scale project for the storage of CO₂ in a subterranean aquifer. CO₂ is stored in supercritical state 250 km off the Norwegian coast. About one million tons of CO₂ is removed from produced natural gas and subsequently injected underground, annually. CO₂ injection started in October 1996 and by 2008, more than ten million tons of CO₂ had been injected at a rate of approximately 2700 tons per day. The formation into which the CO₂ is injected is a brine-saturated unconsolidated sandstone about 800-1000 m below the sea floor. A shallow long-reach well is used to take the CO₂ 2.4 km away from the producing wells and platform area. The injection site is placed beneath a local dome of the top Utsira formation. Since all CO₂ is injected at approximately the terminal end of the long reach well, the CO₂ is not efficiently distributed over large areas of the receiving Utsira Formation. Thus the capacity of the subterranean geological formation is not used to its full extent.

The In Salah CCS Project is an onshore project for the production of natural gas from a gas reservoir located in a subterranean aquifer. The aquifer is located in the Sahara desert. The reservoir is in a carboniferous sandstone formation, 2000 m deep. It is only 20 m thick, and of generally low permeability. Natural gas containing up to 10% of CO₂ is produced. CO₂ is separated, and subsequently re-injected into the water-filled parts of the reservoir. The project uses four production and three injection wells. Three long-reach horizontal wells with slotted intervals over 1 km are used to inject 1 MtCO₂ per year. The amount of CO₂ injected through the slotted intervals depends from the local permeability of the formation at the respective slotted intervals. Since the permeability is not constant, more CO₂ is injected through slotted intervals in some areas (having higher permeability than others) than through the slotted intervals in other areas. Hence, an uneven distribution of the injected mass flow results. Furthermore, this uneven distribution of CO₂ injection leads to a significant pressure drop at the interior of the injection well in these areas. This, in turn leads to an even lower rate of injection at the (more distal) regions of low permeability of the geological formation. This adds to the uneven distribution of CO₂ injection of the horizontal length of the well.

U.S. Pat. No. 5,503,226 mentions injection of fluid into geological formations. It discloses a process for recovering hydrocarbons from a subterranean formation having low permeability matrix blocks and high permeability matrix blocks. Hot light gas (in one embodiment, CO₂ gas) is injected through an injection well into the formation to heat the matrix blocks, and to create and enlarge a gas cap in a fracture network, and ultimately to liberate significant portions of the hydrocarbons present in the low permeability matrix blocks. In one embodiment an injection/production well is used which comprises a vertical section and a horizontal section. The vertical section is cased, while the horizontal section is completed open hole. CO₂ is injected into the horizontal open hole section through the terminal opening of a tubing string disposed within the well. No means for providing an even distribution of CO₂ injection into the formation over the longitudinal extent of the well is provided. Hence, an uneven distribution of (local) CO₂ injection results.

The prior art has not identified the uneven distribution of the CO₂ injectivity over the horizontal extent of the injection well as being problematic. Until to date, the uneven distribution, in particular the large injectivity at regions of high permeability, was merely seen as an advantage, because this maximizes the current flow of CO₂ into the formation.

However, simulations conducted by the present inventors have now shown that the non-uniform distribution of CO₂ injection into the formation has negative effect on the overall usage of the storage capacity of the formation. Without wishing to be bound by theory, it is believed that the uneven distribution of the injected amount of CO₂ leads to a situation in which large amounts of CO₂ are stored in the high permeability regions of the formation, while the low permeability regions remain effectively unused. Furthermore, a significant pressure drop in areas of high permeability has shown to lead to inefficient injection through the more distal parts of the injection well.

In view of the above discussed shortcomings of the prior art technology, it is now an object of the present invention to provide arrangements and methods for the permanent storage of CO₂ in subterranean formations, which arrangements and methods allow for a more complete use of the available storage capacity of the formation. It is an object of the present invention to provide arrangements and methods for storing a greater amount of CO₂ in a formation of a given size and capacity. It is an object of the present invention to provide methods and arrangements which efficiently use of the storage capacity of geological formations having significant differences in the permeability at different locations within the formation.

As will be apparent from the description of the invention hereinbelow, methods and arrangements of the present invention rely, to some extent, on hardware components and technology which has already been applied in other technical contexts, for different technical purposes within the drilling industry.

Such methods and technology will now briefly be described.

GB 2325949 discloses a method for obtaining equalized production from deviated production wells comprising a plurality of spaced apart flow control devices. Each control device includes a flow valve and control units to control inflow of oil into the production well. The fluid from various zones are drawn in a manner that depletes the reservoir uniformly along the entire length of the production well. GB 2325949, however, is not concerned with the injection of fluids, in particular CO₂, into geological formations. The use of flow control devices for the injection of fluids into formations is not envisaged or suggested. Nor is there any indication that the disclosed flow control devices would be suitable for injection, in particular for CO₂ injection.

GB 2376488 discloses an apparatus and method for controlling fluid production in a deviated production well which comprises a plurality of inflow control valves. The valves are self regulating or selectively controllable, and they maintain a substantially constant pressure drop between the exterior and the interior of the flow pipe. Application of the controlling devices for CO₂ injection, or the suitability of the inflow control valves, is not shown or suggested.

U.S. Pat. No. 5,141,054 discloses a well completion method for steam stimulation of vertical and horizontal oil production wells. Steam is injected through multiple perforations of controlled size, and used for lowering the viscosity of the viscous hydrocarbonaceous fluids in the vicinity of the horizontal well. The method seeks to achieve a uniform heating along a desired length of the horizontal well. Storage of the injected fluid, let alone, increasing the storage capacity of the formation for such fluids, is not envisaged or taught.

U.S. Pat. No. 5,826,655 discloses a method and an apparatus for enhanced viscous oil recovery. A horizontal well is drilled through a viscous oil formation, and a specially designed steam injection tube, with multiple holes, is used to evenly inject the steam into an outer lumen of the horizontal wellbore. The multiple holes in the steam injection tube are each provided with a sacrificial impingement strap, in order to avoid direct impingement of steam on the slotted liner, and thus, to prevent early erosion of the slotted liner. Steam enters the geological formation not through these multiple holes, but through the slots of the conventional slotted liner, provided around the injection tube. Since the steam can freely move in the annular lumen, laterally outward the injection tube and laterally inward the slotted liner, nothing prevents the steam to enter the geological formation preferentially in parts of the formation having high permeability for steam.

WO 2008/092241 discloses a method for enhanced oil recovery, in which method steam is distributed and injected through perforations into an annular space between an inner tubing and an outer slotted liner in a horizontal injection well. The steam is then injected from the annular space into the oil containing geological formation through slots in the conventional slotted liner. The inner tubing string is provided with multiple ports having a selected distribution and geometry. This causes the steam to be injected into the annular space in a defined manner. Injection of steam into the geological formation is additionally controlled by varying the cross sectional area of the annular space between the inner tubing and slotted liner, such that the axial flow resistance in the annular space is controlled. In one embodiment, the perforated tubing is placed directly in an open hole well bore. Methods for injecting CO₂ into subterranean formations, let alone methods for increasing the available storage capacity of subterranean reservoirs, are not envisaged. It is likewise not foreseen that injecting fluids evenly over the entire horizontal extent of an injection well would maximize the amount of CO₂ storable in a particular formation.

US2009008092 A1 discloses various inflow control devices for use in oil production. The inflow control devices include a plurality of openings that each provide a flow path to the interior of the production tube. It is not disclosed that the disclosed devices can be used in the reversed flow directions, nor is it likely that they are suitable for controlling the flow of less viscous fluids, such as CO₂.

WO 2009/088293 discloses a method for self-adjusting the flow of fluid through a valve or flow control device in injectors in oil production.

U.S. Pat. No. 5,435,393 A discloses a method for production of oil or gass from an oil or gas reservoir and a production pipe for injection of fluids into an oil or gas reservoir.

SUMMARY OF THE INVENTION

The present invention relates to a arrangement for injecting CO₂ in a supercritical state into a subterranean geological formation, said arrangement comprising a conduit having a proximal portion and a distal portion, at least part of said distal portion extending in a substantially horizontal direction; multiple openings being provided in said distal portion of said conduit for injection of CO₂ into said geological formation; wherein at least one, or all, of said multiple openings is/are provided with outflow limiting means for limiting the flow rate of CO₂ through the respective opening into said geological formation.

In a preferred embodiment, said multiple openings are provided in a lateral surface of said conduit.

In a further preferred embodiment the strength of the outflow limiting means in reducing the outflow between each two neighboring outflow limiting means is decreasing along the length of said conduit in the direction towards the distal end.

In a further preferred embodiment the distance between each two neighboring outflow limiting means is decreasing along the length of said conduit in the direction towards the distal end.

In a further preferred embodiment at least one said outflow limiting means is adjustable.

In a further preferred embodiment said conduit is a branched conduit comprising a primary branch and at least one secondary branch. The at least one secondary branch preferably branches off said primary branch in a branch point, said branch point being provided with branch-flow controlling means for limiting the flow of CO₂ into the respective secondary branch. In this embodiment, it is preferred that said at least one secondary conduit is substantially horizontal.

In a further preferred embodiment said secondary conduit that branches off said primary branch in a branch point near a distal end of the said main conduit can be left as open hole.

In preferred arrangements of the invention the conduit is closed at its distal ends. Alternatively, a distal end portion of the conduit is left as open hole.

In a preferred embodiment, said arrangement comprises pressure producing means for producing a pressure in said conduit sufficient for injection of CO₂ in a supercritical state into said geological formation. The arrangement may comprise a source of CO₂. The pressure producing means may be, e.g., a pump, a pressurized CO₂ container, or a pressurized CO₂ pipeline.

In a further preferred embodiment said outflow limiting means comprises at least one capillary fluidly connecting an inner lumen of said conduit with said geological formation.

In a further preferred embodiment said capillary opens at its proximal end towards an inner lumen of said conduit, and opens at its distal end into the geological formation.

In a further preferred embodiment said capillary is a helical capillary, coiled around, and laterally outward, an inner surface of said conduit.

In a further preferred embodiment said capillary has a circular, a triangular, a rectangular or a quadratic cross sectional area. The capillary has preferably a cross sectional area of 10 mm² to 500 mm². Preferably, the capillary has a length of from 10 cm to 500 m, from 10 cm to 200 m, or preferably from 1 m to 100 m. Preferably, the length of the capillary is more than 5 times, 10 times, 20 times, 100 times, or 1000 times larger than the largest diameter of the capillary. Preferably, the length of the capillary is more than 5 times, 10 times, 20 times, 100 times, or preferably 1000 times larger than the square root of the largest cross-sectional area of the capillary.

Preferably, the geological formation is an aquifer, or a confined aquifer, or a closed aquifer.

In a further preferred embodiment said arrangement is for the permanent storage of CO₂ in said geological formation.

The present invention also relates to the use of arrangements of the invention for CO₂ injection into subterranean geological formations.

The present invention also relates to methods for storing CO₂ in subterranean geological formations using the arrangements described above.

The invention thus also relates to a method for storage of CO₂ in a subterranean geological formation, said method comprising: introducing CO₂ into a conduit having a proximal portion and a distal portion, at least part of said distal portion extending in a substantially horizontal direction; wherein multiple openings are provided in said distal portion of said conduit, each provided with flow limiting means for limiting the flow rate of CO₂ through each said multiple openings into said geological formation, and injecting said CO₂ in a supercritical state through said multiple openings into said geological formation.

In preferred methods of the present invention, an arrangement as described hereinabove is used.

SHORT DESCRIPTION OF THE FIGURES

FIG. 1 shows a first embodiment of the invention.

FIG. 2 shows a second embodiment of the invention.

FIG. 3 shows flow limiting means according to one aspect of the current invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to methods and arrangements for the permanent storage of CO₂ in subterranean geological formations.

An “aquifer”, within the context of the present invention shall be understood as being an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay). An aquifer may be sealed by an aquitard or aquiclude at an upper or lower boundary. Such aquifers are hereinafter referred as “confined aquifers”. An aquifer may also be sealed at both the upper and lower boundary. Such aquifers are hereinafter referred to as “closed aquifers”. Preferred aquifers, according to the invention, are upwardly convex aquifers, or downwardly convex aquifers. The “aquifer”, within the context of the present invention, may also be referred to as the “reservoir”.

“Flow limiting means”, in the context of the present invention, shall be understood as being any means that is suitable for limiting the mass flow of fluid through an opening or conduit, preferably in a defined manner. Preferred flow limiting means comprise elongated conduits of a relatively small diameter, e.g., a capillary. Preferred capillaries have a circular, elliptic, rectangular, or quadratic cross-sectional area.

A “capillary”, according to the present invention, shall be understood as being an elongated channel. The use of the expression “capillary” is not to imply that the capillary confers its pressure reducing effect entirely by so-called “capillary forces”. A pressure drop along the length of a capillary of the present invention preferably stems from the friction of fluids moving along the elongated channel of the capillary.

An “openhole”, or a “well completed open hole”, shall be understood to relate an uncased portion of a well, i.e., the well in a state when it is drilled, with no casing, liner, or similar, provided at its outer circumference.

“Permeability” of a formation, in the context of the present invention, is the property of the formation to transmit fluids in response to an imposed pressure difference. Permeability is typically measured in darcies or millidarcies. [Converted to SI units, 1 darcy is equivalent to 9.869233×10-13 m² or 0.9869233 (μm)². This conversion may be approximated as 1 (μm)².] Formations that transmit fluids readily, such as sandstones, are described as permeable and tend to have many large, well-connected pores.

Impermeable formations, such as shales and siltstones, tend to be finer grained or of a mixed grain size, with smaller, fewer, or less interconnected pores.

“Substantially horizontal”, in the context of the present invention, shall mean at an angle of between 45°-135°, or 80°-100°, or 85°-95°, or 90° from the vertical. “Substantially vertical”, in the context of the present invention, shall mean at an angle of less than 45°, less than 20°, less than 10°, less than 5°, or 0° from the vertical.

The present invention is based on the unexpected finding that the available storage capacity of a geological formation for CO₂ can most effectively be used, if the CO₂ is injected from multiple injection points along the length of a long-reach horizontal well in such a way that the mass flow of CO₂ into the formation is approximately constant over the entire length of the horizontal well. While previously, a common wish in the art has been to inject large amounts of CO₂ in as short a time period as possible, the inventors of the present invention have taken a very different approach. By limiting the radial mass flow of CO₂ to a certain maximum value, the present invention produces a substantially even distribution of the radial mass flow over major parts of the horizontal extent of the injection well. This leads to a reduced radial mass flow [kg/s] into the formation, but this obstacle is more than outweighed by the fact that the total amount of CO₂, which can be stored in a particular formation, is dramatically increased.

FIG. 1 generally shows an arrangement of the present invention. Arrangement 1 is used to inject large amounts of CO₂ in the subterranean formation for permanent storage of CO₂ therein. For this purpose, there is provided a conduit 3 which extends from a point above surface down into formation 2 in which the CO₂ is to be stored. The geological formation can be, e.g., a depleted oil field, a depleted gas field, or an aquifer. The aquifer is preferably a closed aquifer, or a confined aquifer. The geological formation is preferably more than 500 m under ground. The geological formation is preferably 5 to 1000 m, preferably 20 to 200 m thick.

Conduit 3 comprises a proximal end portion 4 and a distal end portion 5. The distal end portion 5 comprises a generally horizontal portion. The horizontal (distal) portion is preferably provided in form of a long-reach horizontal well, and is preferably between 100 m and 2000 m long. This allows CO₂ to be injected into the formation at multiple injection points over the entire length of the conduit. CO₂ storage is thereby distributed over a large area/volume of the reservoir formation.

The arrangement comprises pressure producing means 10, e.g., a pump, for injecting CO₂ into the geological formation. In other preferred embodiments of the invention, the pressure producing means may be a pressurized CO₂ container, or a pressurized CO₂ pipeline. The CO₂, when injected, is preferably in a supercritical state. Thus, all components of the arrangement must be appropriately designed and constructed such as to be able to sustain the harsh conditions of its operation. Materials must be appropriately chosen to resist the very high pressures and the corrosion, in particular, when the CO₂ injected is not pure CO₂, but contains, e.g., water and/or other corrosive contaminants, such as O₂ or SO₂. Pressure producing means preferably are able to produce pressures of more than 73, 100, 200, 500, or 1000 bar.

Conduit 3 comprises multiple openings 6 a-6 z in a distal portion, through which openings CO₂ is injected into the formation. At least one, but preferably all openings are provided with outflow limiting means 7 a-7 z. The outflow limiting means 7 a-7 z serve to reduce the radial mass flow of CO₂ through the individual openings 6 a-6 z. The radial mass flow is most efficiently reduced in areas of the formation having high permeability. This is due to the fact that the radial mass flow in these areas—without outflow limiting means—would be very large. In areas of the formation having a low permeability for CO₂, the mass flow into the formation is low from the outset. Flow limiting means have little effect in these areas. As a result of the more efficient flow limitation in highly permeable areas, a substantially even mass flow distribution over the entire horizontal length of the injection well is achieved. In other words, the mass of CO₂ injected per unit time and per unit length of the conduit is approximately constant. It is this even distribution of the radial mass flow rate that is believed to produce the unforeseen inventive effect, namely that the available CO₂ storage capacity of the formation can be used to a far greater extent than without radial flow limitation.

FIG. 2 shows a second embodiment of the invention. In this embodiment conduit 3 is a branched conduit. Secondary branch 9 branches off primary branch 8 in branch point 10. Multiple secondary branches 9 may be provided. In one embodiment (not shown), conduit 3 further comprises tertiary branches, or even higher order branches, branching off the respective lower order branches.

In order to be able to control the mass flow of CO₂ into the secondary branches 9, branch-flow controlling means 13 may be provided. Branch-flow controlling means 13 may be in form of a throttle or a valve, preferably a controllable valve.

Outflow limiting means 7 a-7 z may be in form of an elongated capillary. Preferred capillaries have a circular, elliptic, rectangular, or quadratic cross-sectional area. They preferably have a cross sectional area of from 10 mm² to 500 mm², and independently, a preferred length of from 10 cm to 500 m, from 10 cm to 200 m, or from 1 m to 100 m. Preferred capillaries of the invention are helically coiled.

Capillaries of the present invention are preferably designed such that, under operating conditions, they produce a pressure drop along the length of the capillary of from 0.5 bar to 5 bar.

According to another preferred embodiment of the present invention, the outflow limiting means 7 a-7 z can be modified “inflow control devices” (ICDs), e.g., of the type disclosed in US2009008092 A1. It must, however be noted that those ICDs used for oil production are normally not suitable, and need significant modification in order to be useful in the context of the present invention. This is, i.a., due to the fact that the direction of fluid flow through the devices is reversed. Furthermore, the viscosity of fluids in oil production is generally higher than the one of CO₂, e.g., in a supercritical state. Thus, the cross sectional area and/or length of conduits of the ICDs must be appropriately changed. Also the applicable pressure regime is different in methods of the present inventions as compared to oil production. While in methods of the present invention, the pressure in interior of the injection well can be deliberately chosen, e.g., by the appropriate pressure producing means, in oil production the pressure driving the fluid transport is normally determined by the pressure naturally occurring in the reservoir.

It must also be mentioned that the known inflow control devices would normally not be suitable for use in devices and methods of the present invention without significant modification, because of the extremely corrosive nature of the supercritical CO₂ (at least when impurities, such as water or other corrosive gasses are also present). Hence, outflow limiting means of the present invention must be made of highly corrosion resistant materials.

Outflow limiting means 7 a-7 z may be provided in form of an elongated channel or capillary 12. Flow limiting means 7 a-7 z can be adjustable. Adjustment of the outflow limiting effect can be achieved by controlling (reducing or increasing) the cross-sectional area of the elongated channels or capillaries. The flow through flow limiting means 7 a-7 z can also be adjusted by, e.g., controlling the effective length of the elongated channels or capillaries. Alternatively, the flow through flow limiting means 7 a-7 z can be adjusted by changing the shape of the cross-sectional area in channels or capillaries of flow limiting means 7 a-7 z.

In the embodiment shown in FIG. 3, the flow limiting means 7 are in form of a helical capillary 12, wound around, and disposed radially outward, an inner surface of conduit 3. Capillary 12 opens at a first (proximal) end 19 into inner lumen 18 of conduit 3. A second (distal) end 20 of capillary 12 opens into formation 2. Second end 20 may also open into a sand screen or pervious liner (not shown) provided radially outward of conduit 3. Conduit 3 is thus preferably in close contact with the pervious liner. The pervious liner is preferably in close contact with the formation.

Elongated channels or capillaries 12 of a certain length, as opposed to simple holes, are able to effectively control the mass flow of fluid at relatively modest pressures. Therefore, pumps of lower performance and price can be used. Furthermore, operation under lower pressure also reduces erosion of the system's components, thus, the lifetime of the system is increased.

Outflow limiting means 7 a-7 z generally produce a significant pressure drop between their respective first and second ends. For this reason, the pressure required in conduit 4 for inducing a sufficient radial mass flow is significantly higher than with conventional slotted wells. In order to be able to build a sufficiently high pressure in conduit 3, a plug 17 is preferably provided in a distal end of the conduit. In other embodiments, the distal ends of conduit 3 are not provided with a plug. They may be open-hole wells. The latter embodiments may be suitable in situations where the formation has low permeability at the distal ends of conduit 3, or where the horizontal portion 5 is very long.

As depicted in FIG. 3, conduit 3 may comprise multiple impervious segments 15 and multiple outflow segments 16, wherein the multiple openings 6 a-6 z provided only in the outflow segments 16 (i.e., not in impervious segments 15). There may be provided multiple openings 6 a-6 z, or multiple helical capillaries 12, per outflow segment 16.

Outflow segments 16 and impervious segments 15 are preferably provided with a male fitting at one end and with female fitting at the other end. Impervious segments 15 can be fitted to each other, and to outflow segments 16. Likewise, outflow segments 16 can be fitted to each other, and can be fitted to impervious segments 15. A seal 14 is preferably provided between any two connected impervious segments 15 and/or outflow segments 16. Conduit 3 may thus be of modular construction.

Outflow segments 16 (and preferably also impervious segments 15) are preferably in direct contact with the formation, i.e., there is preferably no annular gap or space between the outflow segment and the reservoir material. This is useful for avoiding significant axial flow of CO₂ radially outward conduit 3. In other words, the outer surface of outflow segment 16 (and preferably also the outer surface of impervious segment 15) contacts formation 2.

Alternatively, radially outward conduit 3, e.g., radially outward of outflow segments 16 and/or impervious segments 15, there may be provided a sand-screen or a pervious liner (not shown). The sand-screen or pervious liner preferably contacts the conduit and the formation, such as to prevent significant axial mass flow of CO₂. The pervious liner is preferably of a material having a permeability in the radial direction (for CO₂) which is equal to, or greater than, its permeability for CO₂ in the axial direction.

Reference Numerals

1—arrangement

2—geological formation

3—conduit

4—proximal portion

5—distal portion

6 a-6 z—multiple openings

7 a-7 z—outflow limiting means

8—primary conduit

9—secondary conduit

10—branch point

11—pressure producing means

12—capillary

13—branch-flow controlling means

14—seal

15—impervious segment

16—outflow segment

17—plug

18—inner lumen

19—first opening

20—second opening 

What is claimed is:
 1. An arrangement for injecting CO₂ in a supercritical state into a subterranean geological formation, said arrangement comprising: a conduit having a proximal portion and a distal portion, at least part of said distal portion extending in a substantially horizontal direction; multiple openings being provided in said distal portion of said conduit for injection of CO₂ into said geological formation; wherein said multiple openings are provided with outflow limiting means for limiting the flow rate of CO₂ through respective said opening into said geological formation.
 2. The arrangement of claim 1, wherein the distance between each two neighboring outflow limiting means is decreasing along the length of said conduit in the direction towards the distal end.
 3. The arrangement of claim 1, wherein said conduit is a branched conduit comprising a primary branch and at least one secondary branch.
 4. The arrangement of claim 3, wherein said at least one secondary branch branches off said primary branch in a point, said branch point being provided with branch-flow controlling means for limiting the flow of CO₂ into the respective secondary branch.
 5. The arrangement of claim 1, wherein said conduit is closed at its distal ends.
 6. The arrangement of claim 1, wherein a distal end portion of said conduit is open hole.
 7. The arrangement of claim 1, wherein said arrangement comprises pressure producing means for producing a pressure in said conduit sufficient for injection of CO₂ in a supercritical state into said geological formation.
 8. The arrangement of claim 1, wherein said outflow limiting means comprises at least one capillary fluidly connecting an inner lumen of said conduit with said geological formation.
 9. The arrangement of claim 8, wherein said capillary opens at its proximal end towards an inner lumen of said conduit, and opens at its distal end into the geological formation.
 10. The arrangement of claim 1, wherein said geological formation is an aquifer.
 11. A method for storage of CO₂ in a subterranean geological formation, said method comprising: introducing CO₂ into a conduit having a proximal portion and a distal portion, at least part of said distal portion extending in a substantially horizontal direction; and injecting said CO₂ in a supercritical state through multiple openings into said geological formation, wherein said multiple openings are provided in said distal portion of said conduit, and are each provided with flow limiting means for limiting the flow rate of CO₂ through each said multiple openings into said geological formation.
 12. The method of claim 11, wherein said CO₂ is injected through an arrangement for injecting CO₂ in a supercritical state into a subterranean geological formation, said arrangement comprising: a conduit having a proximal portion and a distal portion, at least part of said distal portion extending in a substantially horizontal direction; multiple openings being provided in said distal portion of said conduit for injection of CO₂ into said geological formation. 